1. Field of the Invention
The present invention relates to a bandpass filter high-frequency module with wideband and steep attenuation characteristics to be used preferably in UWB (Ultra Wide Band) wireless communications fields, and to wireless communications equipment using the same. UWB is expected to be used as data transmission medium for PC peripherals such as PC adaptors, external storage devices, printers, scanners, and hubs or for digital consumer electronics such as digital TVs, projectors, 5.1 ch speaker systems, and video cameras.
2. Description of the Related Art
UWB (Ultra Wide Band) has drawn attention recently as a new communications system.
UWB is a communications system for achieving large-volume data transmission with a pass band of 3.1 to 10.6 GHz.
Comparing UWB with wireless local area networks (hereinafter referred to as W-LAN) for use as one of data communications means, there are differences in communications distance and data transmission rate. W-LANs have a communications distance of 30 to 100 m, transmission power of 500 mW, and communications speed of approximately 11 Mbps, while UWB applications, though having a shorter communications distance of 10 m, allow for lower power consumption with a transmission power of 100 mW and for higher-speed data transmission with a communications speed of 100 Mbps at a communications distance of around 10 m and 480 Mbps at a communications distance of 2 m or less.
The U.S. FCC regulations make some arrangements for the frequency band to be used in UWB applications, and a wide band of 3.1 to 10.6 GHz will be used therein.
As mentioned above, one characteristic of UWB applications is to use a wide band. The relative band (bandwidth/center frequency) thereof is required to be 40% or more, and further 108% in some cases.
Also, the average transmission power density of UWB applications is defined to be a low value of less than −41.3 dBm/MHz. Here, −41.3 dBm/MHz is equivalent to radiation power generating an electric field intensity 54 dBμV=500 μV/m at a distance of 3 m from the wave source.
As mentioned above, another characteristic of UWB applications is to require a lower transmission power.
Meanwhile, the FCC defines the spectrum mask under an outdoor environment to be, for example, −20 dB at 3.1 GHz and −30 dB at 1.61 GHz using the transmission power within a pass band of 3.16 to 4.75 GHz as a reference (0 dB). It is also necessary to take account of the impact with W-LANs (802.11.a) under practical service conditions, requiring attenuation at 5.15 GHz.
Therefore, still another characteristic of UWB applications is to require the transmission power spectrum to be attenuated steeply within narrow bands adjacent to the pass band.    [Related Art Document 1] K. Li, K. Kurita, and T. Matsui, “An ultra-wideband bandpass filter using broadside coupled microstrip-coplanar waveguide structure,” IEEEMTT-S Sym., WE2F, June 2005.
For the foregoing reasons, filters inserted in the pathway of transmitting and received signals in UWB wireless communications equipment are required to have wideband, low-loss, and highly attenuating characteristics near the pass band.
Meanwhile, planar circuit filters have been employed using a dielectric substrate as well-used filters.
FIG. 45 is a perspective view showing a planar circuit filter in which two microstrip lines 31 and 32 are arranged side by side on a dielectric substrate 33.
The two microstrip lines 31 and 32 are arranged side by side on the same wiring layer with one for input and the other for output, and the long sides of the lines are brought close to each other to be coupled. Such a coupling by arranging two resonators side by side on the same plane is a so-called “edge coupling.” This coupling causes a resonance to achieve a narrowband filter.
However, since the two microstrip lines 31 and 32 are arranged side by side on the dielectric substrate in the planar circuit filter, there can be no strong coupling, resulting in difficulty in achieving a wideband filter having a relative bandwidth of 110%. It is also difficult to achieve steep attenuation characteristics. Forming an attenuation pole to improve the attenuation characteristics causes the circuit configuration to be complicated, also resulting in an increase in size. Therefore, the foregoing structure can be said to be not so suitable for a small-sized bandpass filter for UWB applications.